In a manufacturing process of a semiconductor device or a FPD (Flat Panel Display), plasma is used to perform a process, such as etching, deposition, oxidation or sputtering, so as to perform a good reaction of a processing gas at a relatively low temperature. Conventionally, plasma generated by a high frequency electric discharge in MHz frequency band has been used in this kind of plasma process. The plasma generated by the high frequency electric discharge is largely divided into capacitively coupled plasma and inductively coupled plasma according to a plasma generation method (in view of an apparatus).
Generally, in an inductively coupled plasma processing apparatus, at least a part (for example, a ceiling) of walls of a processing chamber may have a dielectric window, and a high frequency power is supplied to a coil-shaped RF antenna positioned at an outside of this dielectric window. The processing chamber serves as a depressurizable vacuum chamber, and a target substrate (for example, a semiconductor wafer and a glass substrate) is provided at a central region within the chamber. A processing gas is supplied into a processing space formed between the dielectric window and the substrate. A high frequency AC magnetic field having magnetic force lines is generated around the RF antenna by a high frequency current flowing in the RF antenna. The magnetic force lines of the high frequency AC magnetic field are transmitted to the processing space within the chamber via the dielectric window. As the RF magnetic field of the high frequency AC magnetic field changes with time, an inductive electric field is generated in an azimuth direction within the processing space. Then, electrons accelerated by this inductive electromagnetic field in the azimuth direction collide with molecules or atoms of the processing gas so as to be ionized. In this process, a donut-shaped plasma may be generated.
Since a large processing space is formed within the chamber, the donut-shaped plasma can be diffused efficiently in all directions (particularly, in a radial direction) and a plasma density on the substrate becomes very uniform. However, only with a conventional RF antenna, the plasma density on a substrate is not sufficiently uniform for most plasma processes. In the plasma process, it is also one of the important issues to improve uniformity of a plasma density on a substrate since a uniformity/reproducibility and a production yield of a plasma process depend on the plasma uniformity.
In the inductively coupled plasma processing apparatus, a characteristic (profile) of plasma density distribution within the donut-shaped plasma formed in the vicinity of the dielectric window within the chamber is important. Especially, the profile of plasma density distribution affects characteristics (especially, uniformity) of plasma density distribution on the substrate after the diffusion of the plasma.
In this regard, there have been proposed several methods for improving uniformity of plasma density distribution in a circumferential direction by dividing the RF antenna into a multiple number of circular ring-shaped coils each having different diameter. There are two types of RF antenna division methods: a first type of connecting the multiple number of circular ring-shaped coils in series (see, for example, Patent Document 1) and a second type of connecting the multiple number of circular ring-shaped coils in parallel (see, for example, Patent Document 2).
Patent Document 1: U.S. Pat. No. 5,800,619
Patent Document 2: U.S. Pat. No. 6,288,493
In accordance with the first type method among the aforementioned conventional RF antenna division methods, since an entire coil length of the RF antenna is large as a sum of all the coils, a voltage drop within the RF antenna may be fairly large and not negligible. Further, due to a wavelength effect, a standing wave of electric current having a node in the vicinity of a RF input terminal of the RF antenna may be easily formed. For these reasons, in accordance with this first type method, it may be difficult to achieve uniformity of plasma density distribution in a diametrical direction as well as in a circumferential direction. Thus, the first type method is deemed to be inadequate for a plasma process for which large-diameter plasma is necessary.
Meanwhile, in the second type method, a RF current supplied to the RF antenna from a high frequency power supply may flow in a greater amount through an inner coil having a smaller diameter (i.e., smaller impedance), whereas a relatively small amount of RF current may flow through an outer coil having a larger diameter (i.e., larger impedance) within the RF antenna. Accordingly, plasma density within the chamber may be high at a central portion of the chamber in a radial direction while the plasma density may be low at a peripheral portion thereof. Thus, in the second type method, capacitors for adjusting impedance are additionally coupled to the respective coils within the RF antenna so as to adjust a split ratio of the RF current flowing through the respective coils.
In such a case, if a capacitor for adjusting impedance is provided on a return line or an earth line of the high frequency power supply, i.e., on an end of the RF antenna, an electric potential of a coil may become higher than a ground potential, so that a sputtering effect causing damage and degradation of the dielectric window by ion attack from the plasma can be suppressed. However, since the coil of the RF antenna is electrically terminated through the capacitor, a length of an equivalent short-circuit resonance line is shortened. As a result, a wavelength effect may easily occur in the outer coil having the larger diameter (length). Therefore, there may occur the same problem as mentioned in the first type method.